The present invention relates to filters in general and in particular to a method and system for estimating the bandwidth of an integrated filter.
Cellular telephones, as with most communication systems, require high gain baseband filters within the receive signal path. In such applications, the in-band signal is amplified and conveyed to subsequent stages for processing, e.g., to an analog-to-digital converter (ADC). This analog filtering serves two purposes: reducing the magnitude of interfering signals outside the band of interest; and providing anti-aliasing.
Continuous-time filters have become widely used in commercial applications. Two main categories of filters are currently used, the Gm-C filters using transconductors and capacitors or the active RC filters constructed from resistors, capacitors, and integrated amplifiers. A drawback of the existing filters used in VLSI applications is their sensibility to the manufacturing process and temperature variations, which may yield to a variation of the nominal value of the Gm or the RC product up to +/xe2x88x9250%. Consequently the bandwidth of the filter may also vary, and it has become necessary to tune the frequency response of the filters to compensate for these variations. However, in order to implement an accurate compensation system, it is appropriate to make a fine measurement of the filter bandwidth.
Several solutions have been proposed to measure the bandwidth of a filter. A first prior art uses an external clock system directly on the manufacturing line.
A second prior art that requires a fine clocking allows to measure the value of the RC product by measuring the charge time of a capacitor through a resistor. The drawback of this solution is the need of an accurate voltage reference.
There exists other methods that compare the oscillation frequencies of an internal and an external RC oscillator. However, these methods use analog circuits which require large silicon surfaces to implement.
In view of the foregoing and other problems of the conventional systems and methods, it is an object of the invention to provide a system for estimating the bandwidth of an integrated filter that is fully digital.
It is another object of the invention to provide a system that is easily integrated on integrated circuits.
These objects are achieved in a preferred embodiment, by a system for estimating the bandwidth of a baseband filter that produces a phase shift on arriving analog signals. The system comprises means for generating a digital reference clock signal and means for converting the digital reference clock signal into an analog reference clock signal to be input to the baseband filter. Phase comparison means are coupled to the baseband filter for comparing the digital reference clock signal to the analog reference clock signal phase shifted through the baseband filter. A digital pulsed signal that is representative of the phase shift is generated, and digital circuit means connected to the phase comparison means convert the digital pulsed signal into a digital value, the digital value being proportional to the phase shift of the baseband filter.
These and other aspects of the invention are described in further detail below.